Feh!
HTHS is only 4.5, and it will no doubt shear a few points with use. The bearings, pistons, rings, etc. will barely notice a <50% increase over typical (~3.0) 10w30. Zee Oil (GC) 0w30 is 3.7, Redline 10w30 = 3.8. No one cries "The End is Near" when people use those fine oils.
Barring extreme cold weather, M1 15-50 won't hurt a thing. The stuff is designed for flat tappet applications, with significantly more Phos/Zinc and, AFAIK, Boron, than most other M1 mixes. I've got a small stash of EP, and add a Qt to the usual 10w30 my Accord runs. It also gets used as a mid-change top off, about 1/2 qt at 4K.
It's a shame Mobil will be discontinuing the oil in May; I've set aside one of my $10 rebate forms in the hopes of snagging some at closeout prices.
Having said all that, I've got to admit that I'd be a little shy of using it straight. I wouldn't lose too much sleep running 100%, but would prefer to throw in a Qt of 0W20 if filling my Honda. :)